Fibre coating method and apparatus

ABSTRACT

A fiber unit coating apparatus includes a chamber into which an uncured resin coated fiber unit is passed. Microspheres mixed with air are fed into the chamber via ducts and apertures. As a result rapid and even application of the microspheres can be achieved, and the system allows controllable application of the microspheres by varying the rate of flow of the air/microsphere mixture. Positive pressure chambers are provided to prevent the microspheres from blocking the inlet and outlet points where the fiber unit enters and leaves the chamber.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/513,883 filed Nov. 9, 2004 (now issued U.S. Pat. No. 7,618,676),which was the national phase of PCT/GB 03/02011 filed May 9, 2003, andwhich claimed priority from GB Application No. 0210760.5 filed on May10, 2002, the disclosures of which priority applications areincorporated herein by reference.

BACKGROUND

1. Technical Field

The invention relates to a fibre coating method and apparatus inparticular for coating optical fibres.

2. Related Art

It is known to protect optical fibres by laying them in ducts. Onewell-known approaching to laying the fibres in ducts comprises fibreblowing (or “blown fibre”). European patent application no. 0108590,commonly assigned herewith describes a fibre blowing technique in whichthe fibre is blown into a duct on a cushion of air, relying on viscousdrag to advance the fibre.

In order to further protect the fibres conventional resin coatings suchas silicone coatings or UV cured acrylate polymer are known, providingprotection from damage and micro cracks. It has been found thatembedding glass microspheres or other particulate matter into a resincoating on the fibres such as an acrylate polymer provides particularlygood results for blown fibre. The coating is applied to an optical fibreunit comprising either a single fibre or a bundle of fibre.

European patent application no. 0521710, commonly assigned herewith,describes an improved method of coating an optical fibre unit so as toobtain better viscous drag and lower friction in a blown fibre process,where particulate matter such as microspheres typically between 10 and200 μm diameter are embedded in the resin coating. In the process, fibreis fed from a drum through a first resin curing system, a further resincoating is applied, the fibre unit passes through a fluidised/aeratedmass of microspheres (preferably electrostatically charged for uniformcoating) in a through passage. The microspheres adhere to the resincoating and the coating is UV cured.

A problem with this known arrangement is that the fibre inlet and outletcan become blocked with particles requiring downtime to clear theblockage. This can be a particular problem when the microspheres areelectrostatically charged as they are attracted to other surfaces thanthe fibre unit surface.

U.S. Pat. No. 5,851,450 describes an arrangement in which turbulence isintroduced in the through passage to obtain an even distribution ofmicrospheres in the resin coating. The microspheres enter through ahorizontal duct, are deflected downwardly to introduce turbulence andfurther turbulence is induced by angled ribs in the through passage. Inaddition the fibre inlet and outlet are shaped so as to deflect theparticles from the inlet and outlet. A problem with this arrangement isthat it, is difficult to adjust the operating properties of it withouteffectively rebuilding the apparatus as its operation is highlydependent on the configuration of the through passage.

A further problem with known arrangements is that the throughput speedfor coating the fibre is limited, with the apparatus unable to reachspeeds of the order of 300 m/min without severely affecting theuniformity of distribution of the embedded microspheres.

Further problems with known arrangements are that in some cases it maybe difficult to ensure that there is no leakage of microspheres fromknown apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of examplewith reference to the figures of which:

FIG. 1 is a block diagram of components of a fibre coating system;

FIG. 2 is a block diagram of a fibre coating unit according to a firstembodiment;

FIG. 3 is a block diagram of a fibre coating unit according to avariation of the first embodiment;

FIG. 4 is a block diagram of a positive pressure chamber for use infibre coating apparatus;

FIG. 5 is a schematic view of a fibre coating unit according to a secondembodiment;

FIG. 6 is a cut-away view along the line A-A of FIG. 5;

FIG. 7 is a cut-away view along the line B-B of FIG. 5;

FIG. 8 is a schematic view of a fibre coating unit according to a thirdembodiment;

FIG. 9 is a cross-section view along the line C-C of FIG. 8; and

FIG. 10 is a cut-away view along the line D-D of FIG. 8.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The materials, apparatus components and processes for advancing andcoating optical fibres will be well-known to the skilled person and aredescribed in overview only here, with reference to FIG. 1. The apparatuswhich is designated generally 10 includes an optical fibre unit 12comprising either single fibre or a bundle of fibres drawn from a drum(off-line operation) or formed directly (on-line operation) upstream(not shown). The fibre is drawn in the direction designated by arrow A.One or more directional rollers 14 are provided such that the directionof the drawing of the fibre unit 12 can be easily controlled. The fibrepasses through a resin coating element 16 and from there into a microsphere coating unit 18 in which the fibre unit is coated withmicrospheres. An electrostatic gun (not shown) may be associated withthe coating unit 18 to charge the microspheres and hence improve theirattraction to the fibre unit 12. In addition positive pressure chambersare preferably placed at the fibre inlet and outlet of the coating unit18 to prevent leakage of the microspheres. A range of alternative microsphere coating units 18 are described below in more detail and form thebasis of the present invention.

The coated fibre unit 12 then passes into a UV curing unit 20 and fromthere to any appropriate downstream treatment units.

Although the fibre unit 12 is shown being coated in a verticalorientation, dependent on the coating regime imposed at the coating unit18, it can equally run horizontally during the coating process with theprovision of appropriate directional rollers 14.

The optical fibre itself can be of any appropriate type as can thecoating, and in particular an example of an appropriate coating is UVcurable acrylate polymer. Any appropriate particulate matter can beembedded in the coating in the coating unit 18 although preferablymicrospheres of solid glass and diameter typically between 10 and 200 μmare applied. The feed rate of the fibre unit 12 through the apparatuscan exceed 300 m/min. Throughout the present description like referencenumerals refer to like parts.

A first embodiment of the fibre coating unit 18 is shown in FIG. 2. Theuncured resin coated fibre unit 12 passes through a coating chamber 190,which comprises a generally cylindrical glass tube 191 terminating ateach end in a substantially solid block (199 a and 199 b) which eachdefine a chamber having a frusto-conical surface (192 and 193). Thefibre unit 12 also passes through positive pressure chamber 72 locatedadjacent the first end 192 and positive pressure chamber 74 locatedadjacent the second end 193.

A mixture of air and microspheres is admitted into the coating chambervia two ducts 196 passing through block 199 a and apertures 194 alocated in the frusto-conical surface 192. Although in the embodimentthere are only two ducts and apertures, it is understood that the airand microsphere mixture may be admitted using a plurality ofducts/apertures entering either through the walls of the block 199 a oralternatively directly into the tube 191 (as shown later in FIG. 3). Theair and microsphere mixture is created either by fluidising themicrospheres in a hopper (not shown) or mechanical metering method. Themicrospheres are then entrained with the flow of air which is pushedalong ducts 196 and into the glass tube 191.

The air and microsphere mixture distributes through the tube 191,causing microspheres to contact and adhere to the uncured resin coatedsurface of the fibre unit 12. The flow of air and entrained microspherespasses along the tube 191 and exits through scavenging outlet 195 whichcollects the unused microspheres which have not adhered to the fibreunit 12. The collected air and microsphere mixture is recycled for lateruse by re-entry via ducts 196 and apertures 194 a.

A filter membrane 197 extends across the entire cross-sectional area ofthe first end 192, except for an area through which positive pressurechamber 72 containing the fibre unit 12 protrudes. Membrane 197 servesto prevent any microspheres from exiting the chamber through an air exitduct 198 which acts as a pressure relief by allowing air to leave thechamber as necessary as indicated by arrow A. In addition, air may beintroduced through duct 198 and membrane 197 to aid the flow ofmicrospheres through the chamber.

A vibration mechanism 200 is attached to the block 199 a which enclosesthe top of the tube 191. Vibration mechanism 200 is used to cause smallvibrations or agitations to permeate through the block and thereforesupply localised vibrations to the section of the chamber enclosed bythe block, which includes the top of the tube, together with the entryapertures 194 a and membrane 197. The vibrations can help prevent theundesirable settling of some microspheres on surfaces, such as forexample the frusto-conical surface 192, and enhances the flow ofmicrospheres through tube 191 by preventing build up within the chamberdefined by block 199 a. The vibration mechanism 200 illustrated in FIG.2 is air-driven although it is understood that any suitable vibrating oragitating means may be used.

The rate of flow of the air and microsphere mixture through ducts 196 iscontrollable, and thereby allows for feedback to change the density ofthe coating of microspheres on the fibre unit 12. A downstream sensor(not shown) can detect the coating density and a control unit can varythe air and microsphere flow accordingly until the desired coating isachieved.

A variation on the first embodiment is illustrated in FIG. 3, in whichlike numerals designate like features. In this variation, the air andmicrosphere mixture is admitted at staggered locations along thecylindrical portion of the glass tube, as indicated by ducts 196 andapertures 194 b.

Referring to FIG. 4, positive pressure chamber 72 is now described inmore detail. The chamber comprises two elongate tubular portions 205 and206, joined by inlet 207 through which pressurized air is introduced.Together these define a channel 208 of varying radius, through which thefibre unit 12 may pass unhindered. Air flow entering the channel fromthe inlet 207 flows in both directions away from the inlet, with thelarger proportion of the air flow occurring towards tube 191. Theproportion of airflow in the different directions is influenced by theinner diameter differential D1>D2 between the two elongate tubularportions 205 and 206 as illustrated in the drawing (where D1 is theinner diameter of the tubular portion 205 closest to tube 191).Alternatively, or in addition, the length differential L2>L1 can be usedto influence the proportions of airflow.

The operation of the positive pressure chamber 72 as described abovegenerates a stream of gas into the tube 191 thereby substantiallypreventing microspheres from entering the pressure chamber 72 andescaping, and operates at a higher pressure than air which conveysmicrospheres within the fibre coating unit. It will be understood that acorresponding pressure chamber 74 at the opposite end of the coatingchamber will operate in a similar manner.

A second embodiment of the fibre coating unit 18 is shown in FIGS. 5-7,and comprises positive pressure chamber 301, coating chamber 302 andpositive pressure chamber 303. The uncured resin coated fibre unit 12passes through these in the direction indicated by arrow 300. An air andmicrosphere mixture within coating chamber 302 causes multiplemicrospheres to adhere to the uncured resin coating of the fibre unit,which is then cured downstream as discussed earlier.

Coating chamber 302 comprises two substantially solid blocks 304 and305, separated by a cylindrical glass tube 191, which together define apassage through which the fibre unit 12 passes. A mixture of air andmicrospheres is admitted into the coating chamber 302 (in the directionsindicated by arrows 308 and 309) via two inlet ducts 306 and 307 formedwithin upper block 304. The two ducts 306 and 307 open into an annularcavity 314 within block 304. The cavity 314 is formed concentricallyaround the passage through which the fibre unit will pass, and opensdirectly into the end of glass tube 191. A cut-away view looking upwardsthrough coating chamber 302 along line A-A is shown in FIG. 6,illustrating the annular cavity 314 with ducts 306 and 307 joining atthe top. The air and microsphere mixture passes through this annularcavity and then enters into glass tube 191.

The air and microsphere mixture may be created either by fluidising themicrospheres in a hopper (not shown) or a mechanical metering method.The microspheres are entrained with the flow of air through ducts 306and 307, cavity 314, and through glass tube 191 where the coating of thefibre occurs. The air and microsphere mixture finally passes through afrusto-conical shaped cavity 310 within lower block 305 formedconcentrically around the passage through which the fibre unit willpass. The unused air/microsphere mixture exits the coating chamber 302via three outlets 311 a, 311 b and 3111 c, and can be recycled for lateruse by re-entry via inlet ducts 306 and 307 if desired. A cut-away viewthrough block 305 along the line B-B is shown in FIG. 7.

The distribution of glass microspheres adhering to the fibre unit 12 canbe readily and quickly adjusted by changing the feed rate of theair/microsphere mixture through inlet ducts 306 and 307. A downstreamsensor (not shown) can detect the coating density or distribution of themicrospheres on the fibre unit 12 after it has exited the fibre coatingunit, and a control unit may be provided to vary the air/microspherefeed rate so as to vary the density or distribution accordingly.

Positive pressure chambers 301 and 303 are provided at either end of thecoating chamber 302, so as to prevent leakage of the microspheres fromthe chamber, and to protect the system from blockage by microsphereswhere the fibre unit enters and leaves the chamber. Pressure chamber 301comprises an elongate cylindrical channel 313 formed by tubing 312,through which the fibre unit 12 will pass. Part way along this channelis an inlet provided by tubing 214, via which pressurised gas isintroduced into channel 313. Part of tubing 312 extends through block304.

Channel 313 is designed to have differing minimum internal diameterseither side of the inlet, with the portion of channel 313 on the side ofthe inlet furthest from the chamber having the smaller minimum internalradius, namely at location 313 a. Pressurised gas entering the channelvia inlet tubing 214 will thus flow preferentially in the directiontowards the coating chamber 302.

Positive pressure chamber 301 is designed such that channel 313terminates substantially concurrently with the end of annular cavity 314through which the air/microsphere mixture flows, thus ensuring that thefibre unit is protected from the microspheres becoming too deeplyembedded in the resin coating.

The design of positive pressure chamber 301, ducts 306 and 307 andcavity 314 together ensure that the flow of air/microsphere mixturethrough ducts 306 and 307 has ceased to be directed substantiallyradially inwards (which could create too great a force of impingingmicrosphere particles on the fibre unit resulting in the microspheresbecoming embedded too deeply and/or damaging the fibre unit), but hasinstead been redirected to flow generally longitudinally along thechamber 302, within tube 191. Thus, the flow of gas/microsphere mixtureas it enters the tube 191 (i.e., the vector sum of the individual flowvectors of the molecules within the gas/microsphere mixture at thatpoint) is directed generally parallel with respect to the longitudinalaxis of the closest portion of fibre unit 12. There is, of course, adistribution of various speeds and directions of the molecules withinthe mixture which enables microspheres to impinge upon and adhere to theresin coating, but the fact that overall the flow is generally parallelto the fibre means that it prevents a significant number of themicrospheres from impinging directly onto the fibre at too great aspeed. This enables the system to be run at very high speeds, increasingthe flow of air/microsphere mixture accordingly without damage to thefibre unit and/or poor coating quality, and achieving speeds typicallybetween 300-500 m/min.

It is understood that corresponding positive pressure chamber 303 at theother end of the coating chamber 302 is similar in design and operationto chamber 301.

The design of the fibre coating unit of the second embodiment alsofacilitates, in operation, easy threading through of fibre unit 12.Threading is achieved by inserting a pole from each end through positivepressure chambers 301 and 303, the poles having mating connections whichmeet centrally within the glass tube 191. The transparent nature of theglass tube allows the user to easily view the mating connecting ends ofthe two poles, which once connected together can be pulled though untilone of the poles (a tube through which the fibre may be threaded)extends throughout the chamber 302. The fibre unit may then be fedthrough the tube, which is then removed.

A third embodiment of the fibre coating unit 18 is shown in FIGS. 8-10,and comprises positive pressure chamber 301, coating chamber 802 andpositive pressure chamber 303. The uncured resin coated fibre unit 12passes through these in the direction indicated by arrow 300. An air andmicrosphere mixture within coating chamber 802 causes multiplemicrospheres to adhere to the uncured resin coating of the fibre unit,which is then cured downstream as discussed earlier.

Coating chamber 802 comprises two substantially solid blocks 804 and305, separated by a cylindrical glass tube 191, which together define apassage through which the fibre unit 12 passes. A mixture of air andmicrospheres is admitted into the coating chamber 802 (in the directionsindicated by arrows 308 and 309) via two inlet ducts 806 and 807 formedwithin upper block 804. The two ducts 806 and 807 open into an annularcavity 814 within block 804 formed concentrically around the passagethrough which the fibre unit will pass. The cavity 814 is shaped to havea frusto-conical outer wall and opens directly into the end of glasstube 191. A cross section view through coating chamber 802 along lineC-C is shown in FIG. 9, illustrating inlet ducts 806 and 807 openingonto annular cavity 814. The air and microsphere mixture passes throughthis annular cavity and then into glass tube 191.

FIG. 9 shows that whilst inlet ducts 806 and 807 are directed generallyradially inwards towards the passage through which the fibre unit willpass, they are in fact slightly offset from the radial position. Thiscauses improved mixing of the air/microsphere mixture as it impinges atan angle on the inner wall of the cavity 814, and also improves the flowof the mixture through the chamber. This allows the system to be run atvery high speeds, increasing the flow of air/microsphere mixture whilststill providing good coating of the uncured resin surface of the fibrewith microspheres.

The air and microsphere mixture may be created either by fluidising themicrospheres in a hopper (not shown) or a mechanical metering method.The microspheres are entrained with the flow of air through ducts 806and 807, annular cavity 814, and through glass tube 191 where thecoating of the fibre occurs. The air and microsphere mixture finallypasses through a frusto-conical shaped cavity 310 within lower block 305formed concentrically around the passage through which the fibre unitwill pass. The unused air/microsphere mixture exits the coating chamber802 via three outlets 311 a, 311 b and 311 c, and can be recycled forlater use by re-entry via inlet ducts 806 and 807 if desired. A cut-awayview through block 305 along the line D-D is shown in FIG. 10.

The distribution of glass microspheres adhering to the fibre unit 12 canbe readily and quickly adjusted by changing the feed rate of theair/microsphere mixture through inlet ducts 806 and 807. A down streamsensor (not shown) can detect the coating against the distribution ofthe microspheres on the fibre unit 12 after it has exited the fibrecoating unit, and a control unit may be provided to vary theair/microsphere feed rate so as to vary the density or distributionaccordingly.

Positive pressure chambers 301 and 303 are provided at either end of thecoating chamber 802, so as to prevent leakage of the microspheres fromthe chamber, and to protect the system from blockage by microsphereswhere the fibre unit enters and leaves the chamber. Pressure chamber 301comprises an elongate cylindrical channel 313 formed by tubing 312,through which the fibre unit 12 will pass. Part way along this channelis an inlet provided by tubing 214, via which pressurised gas isintroduced into channel 313. Part of tubing 312 extends through block804.

Channel 313 is designed to have differing minimum internal diameterseither side of the inlet, with the portion of channel 313 on the side ofthe inlet furthest from the chamber having the smaller minimum internalradius, namely at location 313 a. Pressurised gas entering the channelvia inlet tubing 214 will thus flow preferentially in the directiontowards the coating chamber 802.

Positive pressure chamber 301 is designed such that channel 313terminates substantially concurrently with the end of annular cavity 814through which the air/microsphere mixture flows, thus ensuring that thefibre unit is protected from the microspheres becoming too deeplyembedded in the resin coating.

The design of positive pressure chamber 301, ducts 806 and 807 andcavity 814 together ensure that the flow of air through ducts 806 and807 has ceased to be directed generally radially inwards (which couldcreate too great a force of impinging microsphere particles on the fibreunit resulting in the microspheres becoming embedded too deeply and/ordamaging the fibre unit) but has instead been redirected to flowgenerally longitudinally along the chamber 802, within tube 191. Thus,the flow of gas/microsphere mixture as it enters the tube 191 (i.e. thevector sum of the individual flow vectors of the molecules within thegas/microsphere mixture at that point) is directed generally parallelwith respect to the longitudinal axis of the closest portion of fibreunit 12. There is of course a distribution of various speeds anddirections of the molecules within the mixture which enablesmicrospheres to impinge upon and adhere to the resin coating, but thefact that overall the flow is generally parallel to the fibre means thatit prevents a significant number of the microspheres from impingingdirectly onto the fibre at too great a speed. This enables the system tobe run at very high speeds, increasing the flow of air/microspheremixture accordingly without damage to the fibre unit and/or poor coatingquality, and achieving speeds typically between 300-500 m/min.

It is understood that corresponding positive pressure chamber 303 at theother end of the coating chamber 802 is similar in design and operationto chamber 301.

The design of the fibre coating unit of the third embodiment alsofacilitates, in operation, easy threading through of fibre unit 12.Threading is achieved by inserting a pole from each end through positivepressure chambers 301 and 303, the poles having mating connections whichmeet centrally within the glass tube 191. The transparent nature of theglass tube allows the user to easily view the mating connecting ends ofthe two poles, which once connected together can be pulled though untilone of the poles (a tube through which the fibre may be threaded)extends throughout the chamber 802. The fibre unit may then be fedthrough the tube, which is then removed.

It will be appreciated that aspects of the various embodiments describedabove can be combined together where appropriate, and that the inventioncan be implemented using suitable materials and apparatus as will beapparent to the skilled person.

1. Apparatus for coating a fibre unit with particulate matter, theapparatus comprising: a passage through which a fibre unit travelsduring use, the passage having at least one particulate matter inlet anda central portion length, a gas stream inlet at an end of the passagefor admitting a gas stream which biases the movement of particulatematter to thereby substantially prevent particulate matter fromcongestion at the end of the passage, and a particulate outletconfigured to direct unused particulate matter from the passageway in adirection substantially parallel to fibre unit travel direction. 2.Apparatus as in claim 1 wherein the gas stream inlet comprises: apressurized gas inlet directed into a length of the passage having areduced cross-sectional area, physical characteristics of the inletbeing such as to direct a greater volume of gas flow from the inlettowards the central portion length of the passage.
 3. Apparatus as inclaim 2 where the physical characteristics comprise: a cross-sectionalarea differential along said length of passage such that a largercross-sectional area is experienced by the air flow passing from theinlet towards the central portion of the passage.
 4. Apparatus as inclaim 2 where the physical characteristics comprise: the gas inlet beinglocated such that the air flow passing from the inlet towards thecentral portion of the passage experiences the longer fraction of saidlength of passage.
 5. Apparatus as in claim 2 in which the jet of gasfrom the inlet is directed toward the central portion of the passage. 6.Apparatus as in claim 2 comprising a gas stream inlet at both ends ofthe passage, from which the jets of gas are directed in opposingdirections, both toward the central portion of the passage.
 7. Apparatusas in claim 2 further comprising means for supplying vibrations to achamber through which particulate matter flows from said particulatematter inlet towards said passage.
 8. Apparatus as in claim 7 where thevibrations are applied to a localized section of the chamber. 9.Apparatus as in claim 1 configured so that, in use, the passage isvertically oriented and the particulate outlet is located at a bottomend of the passage.
 10. A method of coating a fibre unit withparticulate matter, said method comprising: feeding a fibre unit througha fibre unit passage defined in a coating chamber, feeding particulatematter into the fibre unit passage, providing a gas stream in the fibreunit passage biasing the movement of particulate matter to substantiallyprevent particulate matter from entering an end of the passage, anddirecting unused particulate matter to exit the passage in a directionsubstantially parallel to fibre unit travel direction.